When to Measure Postmeal Glucose Levels

Of course that depends on what you want to learn from testing after meals. If you want to know the after-meal peak, then you should test about an hour after you eat. The peak will differ a bit with different foods; fats slow down gastric emptying, and liquids pass through faster than solids. It also can differ with different people.

So if you want to know where your own peaks are, you should test every 15 minutes or so starting about 45 minutes after eating and continuing until the numbers start coming down.

If you want to know if you’re able to return to normal, or close to normal, BG levels within a few hours, then you should test at 2 or 3 hours.

“After eating” is also ambiguous. Should you test X minutes after you start to eat or after you stop eating? Again, it depends on your habits. If you wolf your meal down, it doesn’t much matter. If you eat leisurely and take 30 minutes to finish a meal, then it does. What you really want to know is differences between different meals, so the important thing is to test about the same way every time. Don’t compare one meal you ate in 4 minutes with another one that you took an hour to finish. Most people measure the time after starting to eat.

If you ask CDEs or your doctor when to test, they’ll usually tell you to test at 2 hours. This is because most research papers about postprandial (after meal) BG numbers use the 2-hour reading. But this may not be the best.

One researcher, Antonio Ceriello, recently published a paper proposing that it’s time to switch to a one-hour postprandial measurement. He kindly sent me a copy of the full text of the paper.

Ceriello says that there’s evidence that the one-hour measurement has even stronger power than the two-hour measurement for identifying impaired glucose tolerance. He said this number is also related to the risk for cardiovascular complications. In vitro experiments have shown that just one hour of high BG levels is enough to cause endothelial dysfunction that can then lead to coronary vascular disease, as well as reactive oxygen species (strong oxidants), he said.

Endothelial function is worse at one hour than at two hours both during oral glucose tolerance tests and after meals, he said.

If all this isn’t enough, Ceriello said short-term high BG levels can impair beta-cell function.

When I’ve measured BG levels in nondiabetic friends and relatives, I’ve sometimes found one-hour readings of 160 or so, but the numbers come down to baseline by two hours. Testing only at two hours wouldn’t identify these people, who might be at risk of developing diabetes in the future.

Ceriello’s recommendations apply to clinical studies, but there’s no reason you couldn’t measure at one hour if you so chose. The best, of course, would be to measure at both one hour and two hours if you could afford enough strips. Then you’d know which time made most sense for you.

Even if most clinicians accept Ceriello’s recommendations, it will take time before they become standard. So for now, most studies will continue to use the two-hour numbers. But when you see a study mentioning postprandial (or postmeal) numbers in the future, you should check to see what they mean, if you can (abstracts may not specify).

If anyone wants references to the studies Ceriello was citing, let me know and I’ll send them to you. If I get tons of requests, I’ll just edit this post to put the links in.

Gretchen Becker was diagnosed with type 2 diabetes in 1996 and is the author of The First Year: Type 2 Diabetes and PreDiabetes, and co-author of The Four Corners Diet. She has since been devoted to learning about this chronic disease and sharing it to educate other patients through her background in both science and journalism. Gretchen studied biology for 8 years at Radcliffe/Harvard where she was a PhD Candidate, and published two papers in peer-reviewed journals. After hitting the "glass ceiling" in the world of domestic service, she moved on to other jobs including lab technician, newspaper reporter, woodenware salesperson, cheese plant worker, and tax assessor. She lives on a small sheep farm in Vermont and for many years was a freelance editor of medical books and journals. Find more on Gretchen at her website.